The surface density (﹩{{\rm{\Sigma }}}_{\mathrm{eff}}={M}_{* }(\lt {R}_{{\rm{e}}})/\pi {R}_{{\rm{e}}}^{2}﹩) plotted against stellar mass. We show a theoretical limit Σ_eff ≦ 3 × 10⁵ M_⊙ pc⁻², determined by M. Y. Grudić et al. (2019), as the dashed black line. For illustration, we show regions in M_*–Σ_eff space for multiple stellar systems estimated from P. F. Hopkins et al. (2010), the class of star clusters (SCs), ultracompact dwarf galaxies (UCDs), and globular clusters (GCs) (green); local elliptical galaxies (gray); and compact z > 2 elliptical galaxies (yellow). We also show (see the text) quiescent galaxies at z ∼ 4–5 (A. C. Carnall et al. 2023; A. de Graaff et al. 2024; D. J. Setton et al. 2024), compact starburst galaxies at z = 0.4–0.8 (A. M. Diamond-Stanic et al. 2021), Cosmic Gems star clusters at z ∼ 10 (A. Adamo et al. 2024), and the MW nuclear star cluster. The surface density measured for the three Balmer break galaxies are shown for the three sets of stellar masses ﹩{M}_{* ,\min }﹩, M_*,med, and ﹩{M}_{* ,\max }﹩ in orange, red, and purple, respectively.